How can a soldier refuse an OER within two months of retirement?

Due to your 'typo' of "me" in your first sentence, this sounds like an "asking for a friend" type of question ...

Your statement ("...command is trying to give me an OER less than sixty days of retirement (crazy) in which they currently on leave for the remaining time out-processing.") could be read one of two ways:
• An evaluation was completed prior to retirement and now the command wants to do another that is covering the the last 60 days.
• The command wants to do a regular evaluation and either the THRU date ends during the last 60 days prior to retirement or the processing of the evaluation is happening in the last 60 days.

If the first, then you are correct. That's crazy and the system won't let them do it, so I'm going to assume you mean the second interpretation.

In a word, "no", the rated officer can't "opt-out" of having an evaluation written. They can refuse to participate in it (i.e., not sign the OER) in which case it will be processed without the signature (a comment is included stating that they refused to sign, or more often, that the rated officer "was unavailable to sign due to retirement").

However, if either the rater or senior rate want to do one, the rated officer can't stop it from being done. A 'retirement OER' (actually a Change of Duty OER) will be done if the rater, senior rater or rated officer request it (AR 623-3*, para 3-2g(1)).

The "optional" part is assuming it's been less than a year since the THRU date of the last OER. If not, then an annual is required regardless if the THRU date would be the day before they are seperated from the military. The exception to this would be if the THRU date of the last OER was within 90 days of the 'last evaluated' date.

Are your .. err .. the rated officer, concerned that the unit would require you to come in to sign the OER while on terminal leave (it's digital .. they'll be notified at their .mil address) or something else?

Most likely, they aren't trying to "harass" the rated officer and instead using a retiring officer to 'pad the profile' (which is FREQUENTLY done when a Soldier separates). If this is the case, usually the individual is given a average OER so there is one more evaluation in the rater and senior rater profile. This in turn means they have more 'wiggle room' to give a 'top block' OER to someone else without being constrained by their rater/senior rater profile.
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* AR 623-3 (Evaluation Reporting System) - https://armypubs.army.mil/epubs/DR_pubs/DR_a/ARN14342-AR_623-3-000-WEB-1.pdf

I have not once used or needed a single one of my NCOERs in the civilian world. None of them, let alone specifically the final one before I retired.

Command is likely trying to pad their stats - one more OER completed and slightly higher percentage of completed on time. Plus, if things have not changed and raters and senior raters still have profile which restrict the number of top block ratings they can give, this pads their stats and gives them one more "average" OER to make it that much sooner they can issue another top block.

There is zero reason to fight command on this; just help them out.

Am I missing something? What difference does it make? They give you an OER and you retire. OK, you have to fill out your portion, it's not like they can keep you if you don't do it well.